The present invention relates to the field of power amplifiers. More particularly, it relates to a multi-stage radio-frequency (“RF”) power amplifier having a single input port and a single output port with two or more modes of operation at different power level ranges. Each mode has its active output stage presented with a near optimum load impedance to provide an improved trade off between efficiency and linearity.
Personal wireless communication devices are often required to operate at multiple output power levels to maintain their efficiency (i.e., the ratio of signal output power to the total input power). The required output power level may depend on, inter alia, the signal modulation scheme employed or the quality of the communication channel used by the device. Variable-envelope modulation schemes, such as CDMA (Code Division Multiple Access), for example, typically limit output power because of signal distortion occurring at high output power levels and are usually referred to as linear applications, i.e. requiring the employment of a linear amplifier. Constant-envelope modulation schemes, in contrast, have less stringent power limitations because of greater tolerance to signal distortion. In another aspect, a communication device operating in a poor quality wireless channel may require a higher output power level than when operating in a better quality channel. While low power levels better conserve battery power, high power levels usually assure better signal quality.
The efficiency of a wireless communication device is primarily limited by the efficiency of the device's power amplifier because it imposes the largest drain on the battery. For maximum efficiency, power amplifiers are generally designed to operate at their peak output power, and any deviation from the peak output power reduces the amplifier's efficiency. A power amplifier typically includes a power-amplifying transistor and an impedance matching network to facilitate energy transfer from the transistor to its load, such as for example, an antenna. Impedance matching networks are often precisely designed to maximize energy transfer and to improve the efficiency of the power amplifier. For linear applications, the impedance matching networks are optimized at the maximum expected linear output level to achieve the best compromise between linearity and efficiency since these two properties usually have opposing requirements.
Power amplifiers operating at multiple output power levels may be efficiently implemented by connecting in parallel several power amplifying stages having different power capacities. One such arrangement is shown in FIG. 1. The resulting parallel-stage power amplifier 100 comprises two amplifying stages 110 and 120 each having a power amplifying transistor (not shown), and two impedance matching networks 130 and 140. The output power control in parallel-stage power amplifier 100 is accomplished by enabling or disabling particular amplifying stages, thereby efficiently increasing or decreasing the output power level.
A problem with such parallel-stage design is that each amplifying stage can adversely affect the performance of the other. In one aspect, the impedance matching network of the disabled stage may alter the effective impedance of the active stage and reduce its efficiency. In another aspect, the output impedance of an amplifying transistor may vary during operation adversely affecting that of the other transistor. For example, the output impedance of each transistor depends on whether it is enabled (i.e., operating in its active region) or disabled (i.e., cut-off) and the particular bias voltages that are applied. These conditions may cause cross-coupling and power leakage between the stages, so that each stage operates less efficiently than intended.
Efforts to eliminate the above limitations of the parallel-stage power amplifier have focused on designs using quarter-wavelength transformers as impedance matching components 130 and 140 of power amplifier 100 shown in FIG. 1 and described in U.S. Pat. No. 5,541,554 to Stengel et al. A quarter-wavelength transformer is a section of a transmission line that is equal to one-quarter of a wavelength (λ) at the amplifier's operating frequency (f0). See, e.g., Rudolf Graf, MODERN DICTIONARY OF ELECTRONICS (7th ed. 1999). A quarter-wavelength transformer transforms an input impedance Z1, to an output impedance Z2 according to the following equation: Z2=Z02/Z1, where Z0 is the characteristic impedance of the transmission line forming the quarter-wavelength transformer. Stengel et al. exploits this characteristic of a quarter-wavelength transformer to isolate the disabled stage from the active stage. In particular, by saturating the amplifying transistor in the disabled stage, the input to its quarter-wavelength transformer is effectively short-circuited to ground. In turn, the quarter-wavelength transformer transforms the short-circuit at its input (i.e., Z1=0) to an infinite impedance at its output (i.e., Z2=∞), thus, effectively isolating the active stage from the disabled stage.
Designs based on quarter-wavelength transformers, such as that of Stengel et al., are too bulky for the compact and inexpensive personal wireless communication devices favored by consumers. The quarter-wavelength transformer occupies a large portion of space on an integrated circuit because it must be as long as one-quarter of the wavelength of the signal passing through it. For a cellular telephone signal having a center frequency of 1.9 GHz, the quarter-wavelength transformer is about 2.5 cm long for a typical phone board with dielectric constant of about 3.8. It thus consumes a large space in the device if implemented as part of the integrated circuit and requires a large area on the integrated circuit, unduly increasing its size and cost. In another aspect, a quarter-wavelength transformer has a relatively limited frequency response that is too inflexible for communication devices operating in multiple frequency bands.
Another approach is to use switches to improve the isolation and reduce the leakage effects. Switches, however, can also be physically large and impractical unless integrated on the same IC as the power amplifier.